It is often desired to determine the level of a particular liquid in a tank, container, or some other type of vessel. In some applications, a suitable gas (e.g., air, vapor, etc.) may exist in the head space above the liquid. In other applications, another less-dense liquid (e.g., oil) may float on the surface of the particular liquid (e.g., water).
Many techniques exist for measuring or monitoring the level of a liquid in a container. Among non-optical techniques, the most common are the familiar sight gauge, and various float-operated mechanisms.
In recent years, various additional techniques have been developed to optically determine the level of such liquids. According to one technique, a conventional optical fiber is provided with prismatic end in the form of a conical tip which is formed such that the angle of incidence (.theta..sub.i) of light propagated along the fiber and striking the tip surface is greater than the "critical angle" (.theta..sub.c) of such tip material with respect to air, but less than the "critical angle" of such material with respect to the particular liquid whose level is to be sensed. Hence, when the tip is above the surface of the liquid, substantially all of the light transmitted through the fiber will be reflected internally back through the fiber toward a suitable photo-detector. However, when the probe tip is submerged, light will be refracted into the liquid. Hence, the intensity of the light returned to the detector may be used to indicate whether the probe tip is above or below the surface of the liquid. This and other optical techniques are shown and described in Rakucewicz, "Fiber-Optic Methods of Level Sensing", Sensors (December 1986) [at p. 5 et seq.].
While these techniques may work with non-corrosive fluids and/or with some fluids at room temperature, other fluids are known to chemically attack the materials of which many optical components are formed. For example, hydrofluoric acid (HF) will chemically attack glass, and sulfuric acid (H.sub.2 SO.sub.4) will attack sapphire (Al.sub.2 O.sub.3). Hence, optical components made of such materials are unsuited for exposure to these corrosive liquids. Such corrosive fluids are commonly stored in tanks made of a polytetrafluouroethylene perfluouroalkoxy material, commonly known as "Teflon PFA" and manufactured by E. I. du Pont de Nemours & Co. (Inc.), Wilmington, Del. 19898. While this material is relatively immune to chemical attack by such fluids, one can not, as a practical matter, use conventional optical components (e.g., sapphire, glass, etc.) therewith.
Because of this, it has been proposed to formulate specially-configured sensor tips of such "Teflon PFA" material. See e.g., Tregay, "Optical Liquid Level Sensor", International Patent Application No. PCT/US 88/00907 filed 22 Mar. 1988, the aggregate disclosure of which is hereby incorporated by reference, and assigned to the assignee of the present application. While this type of sensor is capable of reliable operation (e.g., can produce a dry-to-wet signal ratio of more than 20:1 for air, water at room temperature) when there is a clear and distinct transition between the liquid and the gas thereabove, additional considerations come into play when the tanks, upon which such sensors are mounted, are heated and/or when heated liquid is pumped into such tanks. For example, vapor may form or condense and/or gas bubbles may collect on the probe tip. Such vapor, bubbles and/or condensation due to temperature differentials can result in the sensor tip outer surface being exposed to a mixed gas/liquid phase. Since the principle of "total internal reflection" versus refraction for a particular probe tip geometry, hinges on the relative indices of refraction of the probe material and the fluid that wets the tip outer surface, the consequence of exposing the tip surface to such mixed gas/liquid phase is that the intensity of the ideally-high "dry" optical return signal falls, thereby reducing the effective dry-to-wet signal ratio. Indeed the dry-to-wet signal ratio appear to vary inversely with temperature. Accordingly, it would be desirable to provide an improved optical liquid level sensor having reduced sensitivity to vapor, films, fog and condensation droplets, and therefore having reduced sensitivity to the temperature of the serviced fluid(s) and other operational considerations.